Obviously the vast majority of Firefox's settings cannot fit on the Settings page. I don't know whether this one will be added there in the future. For now, you can try changing it in Firefox's about:config preferences editor. This worked for me:

(1) In the same or a new tab, tap the address bar and type about:config and press Enter or tap the go arrow. I recommend bookmarking this for future ease of access.

(2) In the Search Settings box, type script and press Enter.

(3) Scroll down to javascript.enabled and tap Toggle to switch it to false. After a moment it should turn bold and show the change.